The present invention relates to the field of power plant technology.
Such a power plant has been disclosed, for example, in U.S. Pat. No. 5,416,398 the entire disclosure of which is hereby incorporated by reference of the applicant.
The invention also concerns a method of operating such a power plant.
FIG. 1 shows a detail of the electrical circuit of a power plant 10 as disclosed by the prior art and as partly described in U.S. Pat. No. 5,416,398. The known power plant 10 includes at least one gas turbine 32, which is connected to an electric generator 15 via a shaft. During normal operation, the generator 15 driven by the gas turbine generates an AC voltage, which, via a then connected generator switch 14, a bus duct 31, a controllable-step-up transformer 13 equipped with a tap changer, and a line switch 12, is fed into a high-voltage network 11 (high voltage HV) for transmission to loads. Some of the power generated by the generator 15 is branched off from the bus duct 31 upstream of the step-up transformer 13 and is delivered via a connecting line 16, an auxiliary transformer 17 and a switch 18 to a medium-voltage network 21 (medium voltage MV), which carries an auxiliary voltage for meeting the demand for electric power for the auxiliary service of auxiliary components of the power plant 10 (for feed-water pumps, cooling-water pumps and the like.)
An auxiliary generator 19 (e.g. a diesel generator) can be connected to the medium-voltage network 21 via a further generator switch 20 in order to supply the medium-voltage network 21 for the auxiliary service with power in special cases. A low-voltage network 29 (low voltage LV), from which power can be drawn, for example, for the electronics of the power plant, is connected to the medium-voltage network 21 via a medium-voltage switch 26, a low-voltage transformer 27 and a low-voltage switch 28.
During start-up of the power plant 10, the gas turbine 32, which initially is at a standstill, has to be started or run up to firing speed. For this start-up or the run-up, the generator 15 is a drive motor for the coupled gas turbine 32. The variable or controllable supply voltage required for supplying the generator/motor 15 during run-up is provided by a static starting device 24. A switch 25 is provided between the static starting device 24 and the generator/motor 15. The static starting device, as described in U.S. Pat. No. 5,416,398, is preferably designed as a frequency converter and converts an AC voltage which is drawn from the medium-voltage network 21 via a switch 22 and a transformer 23. In this case, the auxiliary voltage on the medium-voltage network 21, during the start-up phase, may alternatively be drawn from the high-voltage network 11 via the connecting line 16 or be provided by the auxiliary generator 19.
During normal operation, the auxiliary voltage on the medium-voltage network 21 is drawn from the bus duct 31 via the circuit parts 16, 17 and 18. However, this has various disadvantages; for example, the generator voltage is controlled with reference to the reactive-power demand in the high-voltage network and the current voltage there. As a result, the voltage at the medium-voltage busbar or on the medium-voltage network 21 also varies, since it is connected directly to the generator via the circuit parts 18, 17 and 16. All the electrical loads in the power plant must be designed for the voltage fluctuations. In addition, with the normal voltage tolerances in the high-voltage networks, the transformer 13 is still to be equipped with a tap changer and a closed-loop control or open-loop control. Furthermore, an additional branch is necessary for the provision of the auxiliary voltage on the medium-voltage network 21, this additional branch including the connecting line 16, the auxiliary transformer 17 and the switch 18. In addition, a fire protection wall and an oil sump have to be provided in the region of the auxiliary transformer. At the same time, the branch necessitates a relatively long bus duct 31. Finally, since only coarse correction of the voltage fluctuations on the bus duct 31 is possible, all the electrical equipment in the power plant 10 which is supplied from the medium-voltage network 21 or the low-voltage network 29 has to be of more complicated design for a comparatively large voltage tolerances.
The object of the invention is therefore to conceive and operate a power plant of the type mentioned above in such a way that the above enumerated disadvantages are avoided and that substantial simplifications and savings are obtained, in particular in the design of the plant.
In the present invention, the static starting device, which in the starting phase of the gas turbine supplies the generator is operated as a motor, from the medium-voltage network, is at the same time used in the opposite direction during normal operation in order to, draw the requisite, in particular controlled, auxiliary voltage from the voltage, produced by the generator, for the medium-voltage network. By the use of the static starting device for the various purposes referred to, it is not only possible to dispense with entire circuit parts of the previous power plant, it is also possible, in a simple manner, to at the same time stabilize or correct the auxiliary voltage in such a way that the electrical equipment of the plant can be designed with smaller voltage tolerance and can therefore be of less complicated design.
A first preferred embodiment of the power plant according to the invention includes a static starting device which is designed as a converter which is constructed with power semiconductors and can be operated in both directions. This has the advantage that proven circuits, for example bridge circuits, with proven components, such as, for example, thyristors, may be used, which circuits can be constructed in a favorable manner and ensure reliable operation.
So that the requisite auxiliary voltage for the generator operated as motor is available without restriction in the starting phase, in another preferred embodiment of the invention, to produce the auxiliary voltage of the medium-voltage network during the run-up of the gas turbine, an auxiliary generator, preferably a diesel generator, can be connected to the medium-voltage network.
In a further preferred embodiment, in order to permit the use of electrical equipment with reduced voltage tolerance, means are provided for controlling the static starting device, and these means, during normal operation of the gas turbine, keep the auxiliary voltage produced by the static starting device on the medium-voltage network constant.
The method according to the invention for operating the power plant includes, to run the gas turbine up, the generator switch is kept open and the generator is used as a drive for the gas turbine, the supply voltage required by the generator being produced by, the static starting device from the auxiliary voltage of the medium-voltage network, and in that, after the run-up of the gas turbine, the generator switch is closed and the auxiliary voltage of the medium-voltage network is produced by the static starting device from the generator voltage delivered by the generator with small tolerances within all load ranges.
The auxiliary voltage required during the run-up of the gas turbine is preferably produced by an auxiliary generator connected to the medium-voltage network, and the auxiliary generator is shut down after the run-up has been completed.